package org.newlucene.core.index;

import java.io.IOException;
import java.io.File;

import org.newlucene.core.document.Document;
import org.newlucene.core.store.Directory;

/** IndexReader is an abstract class, providing an interface for accessing an
  index.  Search of an index is done entirely through this abstract interface,
  so that any subclass which implements it is searchable.

  <p> Concrete subclasses of IndexReader are usually constructed with a call to
  the static method {@link #open}.

  <p> For efficiency, in this API documents are often referred to via
  <i>document numbers</i>, non-negative integers which each name a unique
  document in the index.  These document numbers are ephemeral--they may change
  as documents are added to and deleted from an index.  Clients should thus not
  rely on a given document having the same number between sessions. */

abstract public class IndexReader 
{
	protected IndexReader(Directory directory) 
	{
		this.directory = directory;
	}

	Directory directory;

	/** Returns an IndexReader reading the index in the given Directory. */
	public static IndexReader open(final Directory directory) throws IOException
	{
		// load fieldInfos
		FieldInfos fieldInfos = new FieldInfos(directory);

		SegmentInfos infos = new SegmentInfos();
		infos.read(directory);
		if (infos.size() == 1) // index is optimized
		{
			return new SegmentReader(infos.info(0), fieldInfos, true);
		}
      
		SegmentReader[] readers = new SegmentReader[infos.size()];
		for (int i = 0; i < infos.size(); i++)
		{
			readers[i] = new SegmentReader(infos.info(i), fieldInfos, i == infos.size() - 1);
		}
		return new SegmentsReader(directory, readers);
	}

	/** Returns the time the index in this directory was last modified. */
	public static long lastModified(Directory directory) throws IOException
	{
		return directory.fileModified("segments");
	}

	/**
	 * Returns <code>true</code> if an index exists at the specified directory.
	 * If the directory does not exist or if there is no index in it.
	 * <code>false</code> is returned.
	 * @param  directory the directory to check for an index
	 * @return <code>true</code> if an index exists; <code>false</code> otherwise
	 */
	public static boolean indexExists(String directory)
	{
		return (new File(directory, "segments")).exists();
	}

	/**
	 * Returns <code>true</code> if an index exists at the specified directory.
	 * If the directory does not exist or if there is no index in it.
	 * @param  directory the directory to check for an index
	 * @return <code>true</code> if an index exists; <code>false</code> otherwise
	 */
	public static boolean indexExists(File directory) 
	{
		return (new File(directory, "segments")).exists();
	}

	/**
	 * Returns <code>true</code> if an index exists at the specified directory.
	 * If the directory does not exist or if there is no index in it.
	 * @param  directory the directory to check for an index
	 * @return <code>true</code> if an index exists; <code>false</code> otherwise
	 * @throws IOException if there is a problem with accessing the index
	 */
	public static boolean indexExists(Directory directory) throws IOException 
	{
		return directory.fileExists("segments");
	}

	/** Returns the number of documents in this index. */
	abstract public int numDocs();

	/** Returns one greater than the largest possible document number.
    	This may be used to, e.g., determine how big to allocate an array which
    	will have an element for every document number in an index.
	 */
	abstract public int maxDoc();

	/** Returns the stored fields of the <code>n</code><sup>th</sup>
      <code>Document</code> in this index. */
	abstract public Document document(int n) throws IOException;

	/** Returns true if document <i>n</i> has been deleted */
	abstract public boolean isDeleted(int n);

	/** Returns the number of documents containing the term <code>t</code>. */
	abstract public int docCount(Term t) throws IOException;

	/** 
	 * Returns an enumeration of all the documents which contain <code>term</code>.
	 * For each document, in addition to the document number and frequency of the term in that document, 
	 * a list of all of the ordinal positions of the term in the document is available.  Thus, 
	 * this method implements the mapping:

    <p><ul>
    Term &nbsp;&nbsp; =&gt; &nbsp;&nbsp; &lt;docNum, freq, &lt;pos<sub>1</sub>, pos<sub>2</sub>, ... pos<sub>freq-1</sub>&gt;&gt;<sup>*</sup>
    </ul>
    <p> This positional information faciliates phrase and proximity searching.
    <p>The enumeration is ordered by document number.  Each document number is
    greater than all that precede it in the enumeration. */
	public TermPostings termPositions(Term term) throws IOException 
	{
		TermPostings termPositions = termPostings();
		termPositions.seek(term);
		return termPositions;
	}

	/** Returns an unpositioned {@link TermPostings} enumerator. */
	abstract public TermPostings termPostings() throws IOException;

	/** Deletes the document numbered <code>docNum</code>.  Once a document is
    deleted it will not appear in TermDocs or TermPostitions enumerations.
    Attempts to read its field with the {@link #document}
    method will result in an error.  The presence of this document may still be
    reflected in the {@link #docCount} statistic, though
    this will be corrected eventually as the index is further modified.  */
	public synchronized final void delete(int docNum) throws IOException 
	{
		doDelete(docNum);
	}
	
	abstract void doDelete(int docNum) throws IOException;

	/** Deletes all documents containing <code>term</code>.
    This is useful if one uses a document field to hold a unique ID string for
    the document.  Then to delete such a document, one merely constructs a
    term with the appropriate field and the unique ID string as its text and
    passes it to this method.  Returns the number of documents deleted. */
	public final int delete(Term term) throws IOException 
	{
		TermPostings docs = termPositions(term);
		if (docs == null)
		{
			return 0;
		}
		int n = 0;
		try 
		{
			while (docs.nextDoc()) 
			{
				delete(docs.doc());
				n++;
			}
		} 
		finally 
		{
			docs.close();
		}
		return n;
	}

	/**
	 * Closes files associated with this index.
	 * Also saves any new deletions to disk.
	 * No other methods should be called after this has been called.
	 */
	public final synchronized void close() throws IOException 
	{
		doClose();
	}

	/** Implements close. */
	abstract void doClose() throws IOException;
}